U.S. patent application number 17/516895 was filed with the patent office on 2022-06-09 for radar level gauge system and method for reduced lower dead zone.
The applicant listed for this patent is Rosemount Tank Radar AB. Invention is credited to Mikael Eriksson, Johannes Hjorth, Jimmie Soderstrom.
Application Number | 20220178730 17/516895 |
Document ID | / |
Family ID | 1000005984097 |
Filed Date | 2022-06-09 |
United States Patent
Application |
20220178730 |
Kind Code |
A1 |
Eriksson; Mikael ; et
al. |
June 9, 2022 |
RADAR LEVEL GAUGE SYSTEM AND METHOD FOR REDUCED LOWER DEAD ZONE
Abstract
In summary, the present invention thus relates to a method of
determining a level of a product in a tank, comprising generating
and transmitting an electromagnetic transmit signal; guiding the
transmit signal towards and into the product; returning an
electromagnetic reflection signal resulting from reflection of the
transmit signal; receiving, the reflection signal; determining,
based on the reflection signal and a timing relation between the
reflection signal and the transmit signal, an echo signal
exhibiting an echo signal strength as a function of a propagation
parameter indicative of position along the probe; and determining
the level of the surface of the product based on a propagation
parameter value indicative of a first threshold position along the
probe for which the echo signal has reached a predetermined
threshold signal strength, and an offset indicative of an offset
distance along the probe from the first threshold position towards
the second probe end.
Inventors: |
Eriksson; Mikael;
(Vastervik, SE) ; Hjorth; Johannes; (Goteborg,
SE) ; Soderstrom; Jimmie; (Goteborg, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Rosemount Tank Radar AB |
Molnlycke |
|
SE |
|
|
Family ID: |
1000005984097 |
Appl. No.: |
17/516895 |
Filed: |
November 2, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01S 7/2922 20130101;
G01S 13/88 20130101; G01F 23/284 20130101 |
International
Class: |
G01F 23/284 20060101
G01F023/284; G01S 13/88 20060101 G01S013/88; G01S 7/292 20060101
G01S007/292 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 2020 |
EP |
20211631.5 |
Claims
1. A method of determining a level of a product in a tank, using a
radar level gauge system comprising: a transceiver; a probe
arranged to extend towards and into the product from a first probe
end coupled to the transceiver to a second probe end, the probe
comprising a first probe conductor and a second probe conductor
being conductively coupled to each other by a probe termination
arrangement at the second probe end; and processing circuitry, the
method comprising the steps of: generating and transmitting, by the
transceiver, an electromagnetic transmit signal; guiding, by the
probe, the transmit signal towards and into the product; returning,
by the probe, an electromagnetic reflection signal resulting from
reflection of the transmit signal at the surface of the product and
at the second probe end; receiving, by the transceiver, the
reflection signal; determining, based on the reflection signal and
a timing relation between the reflection signal and the transmit
signal, an echo signal exhibiting an echo signal strength as a
function of a propagation parameter indicative of position along
the probe; and determining, by the processing circuitry, the level
of the surface of the product based on a propagation parameter
value indicative of a first threshold position along the probe for
which the echo signal has reached a predetermined threshold signal
strength, and an offset indicative of an offset distance along the
probe from the first threshold position towards the second probe
end.
2. The method according to claim 1, further comprising the step of:
receiving a temperature parameter value indicative of a present
temperature in the tank; and determining the offset based on the
present temperature.
3. The method according to claim 1, wherein the offset is based on
at least one material property of the product.
4. The method according to claim 1, wherein the offset is based on
at least one previously determined echo signal.
5. The method according to claim 1, wherein: the transmit signal
comprises a first pulse train having a first pulse repetition
frequency; and the method further comprises the steps of:
generating, by the transceiver, an electromagnetic reference signal
in the form of a second pulse train having a second pulse
repetition frequency controlled to differ from the first pulse
repetition frequency by a frequency difference; and the echo signal
is determined based on the reflection signal, the reference signal,
and the frequency difference.
6. A radar level gauge system for determining a level of a product
in a tank, the radar level gauge system comprising: a transceiver
for generating, transmitting and receiving electromagnetic signals;
a probe arranged to extend towards and into the product from a
first probe end coupled to the transceiver to a second probe end,
the probe comprising a first probe conductor and a second probe
conductor being conductively coupled to each other by a probe
termination arrangement at the second probe end; echo signal
forming circuitry connected to the transceiver for forming, based
on the reflection signal and a timing relation between the
reflection signal and the transmit signal, an echo signal
exhibiting an echo signal strength as a function of a propagation
parameter indicative of position along the probe; and level
determining circuitry connected to the echo signal forming
circuitry for determining the level of the product in the tank
based on a propagation parameter value indicative of a first
threshold position along the probe for which the echo signal has
reached a predetermined threshold signal strength, and an offset
indicative of an offset distance along the probe from the first
threshold position towards the second probe end.
7. The radar level gauge system according to claim 6, wherein the
probe termination arrangement provides an inductance between the
first probe conductor and the second probe conductor being higher
than 1 nH.
8. The radar level gauge system according to claim 7, wherein the
probe termination arrangement provides an inductance between the
first probe conductor and the second probe conductor being lower
than 30 nH.
9. The radar level gauge system according to claim 6, wherein the
probe termination arrangement comprises an electrically conductive
member attached to the first probe conductor and to the second
probe conductor.
10. The radar level gauge system according to claim 9, wherein the
electrically conductive member is conductively and mechanically
connected to the first probe conductor and to the second probe
conductor by at least one of welding, screwing, riveting and spring
forces.
11. The radar level gauge system according to claim 9, wherein the
electrically conductive member has an extension of at least 10 mm
in a longitudinal direction of the probe.
12. The radar level gauge system according to claim 6, wherein the
probe is a coaxial probe having an inner conductor and an outer
conductor.
13. The radar level gauge system according to claim 6, wherein: the
radar level gauge system further comprises temperature indicating
circuitry for indicating a temperature parameter value indicative
of a present temperature in the tank; and the level determining
circuitry is further configured to determining the first offset
based on the temperature parameter value.
14. The radar level gauge system according to claim 6, wherein: the
transceiver comprises: transmission signal generating circuitry for
generating the transmit signal in the form of a first pulse train
having a first pulse repetition frequency; and reference signal
generating circuitry for generating an electromagnetic reference
signal in the form of a second pulse train having a second pulse
repetition frequency controlled to differ from the first pulse
repetition frequency by a frequency difference; and the echo signal
forming circuitry is configured to form the echo signal based on
the reflection signal, the reference signal, and the frequency
difference.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a radar level gauge system
and to a method of determining a level of a product in a tank.
TECHNICAL BACKGROUND
[0002] Radar level gauge systems are in wide use for measuring
filling levels in tanks. Radar level gauging is generally performed
either by means of non-contact measurement, whereby electromagnetic
signals are radiated towards the product contained in the tank, or
by means of contact measurement, often referred to as guided wave
radar (GWR), whereby electromagnetic signals are guided towards and
into the product by a probe. The probe is generally arranged
vertically in the tank. The electromagnetic signals are reflected
at the surface of the product, and the reflected signals are
received by a receiver or transceiver comprised in the radar level
gauge system. Based on the transmitted and reflected signals, the
distance to the surface of the product can be determined.
[0003] More particularly, the distance to the surface of the
product is generally determined based on the time between
transmission of an electromagnetic signal and receipt of the
reflection thereof in the interface between the atmosphere in the
tank and the product contained therein. In order to determine the
actual filling level of the product, the distance from a reference
position to the surface is determined based on the above-mentioned
time (the so-called time-of-flight) and the propagation velocity
along the probe of the electromagnetic signals.
[0004] In addition to the reflection at the interface between the
atmosphere in the tank and the product (and at other material
interfaces where applicable), there is typically also a reflection
at the end of the probe close to the bottom of the tank. In most
currently available GWR-type radar level gauge systems, this
reflection at the end of the probe prevents accurate determination
of filling levels close to the end of the probe. The filling level
range for which accurate determination of filling levels is
prevented may be referred to as the lower dead zone or blind
zone.
[0005] In an effort to avoid or reduce the lower dead zone for a
coaxial two conductor probe, EP 2 012 098 proposes to inductively
connect the inner conductor and the outer conductor with a spiral
spring at the end of the probe. The inductive connection between
the inner conductor and the outer conductor delays the reflection
(echo) from the probe end and, according to EP 2 012 098, the lower
dead zone can be reduced or even avoided by choosing the inductance
of the connection between the inner conductor and the outer
conductor.
[0006] A higher inductance, however, requires a longer and/or
narrower electrical connection between the inner and outer probe
conductor, which may be difficult to achieve without requiring a
higher precision in the manufacturing and/or sacrificing some
robustness of the probe.
SUMMARY OF THE INVENTION
[0007] In view of the above, it would be desirable to provide for
an improved GWR-type radar level gauge system, in particular a more
robust and/or production-friendly GWR-type radar level gauge system
having a reduced lower dead zone.
[0008] According to a first aspect of the present invention, it is
therefore provided a method of determining a level of a product in
a tank, using a radar level gauge system comprising: a transceiver;
a probe arranged to extend towards and into the product from a
first probe end coupled to the transceiver to a second probe end,
the probe comprising a first probe conductor and a second probe
conductor being conductively coupled to each other by a probe
termination arrangement at the second probe end; and processing
circuitry, the method comprising the steps of: generating and
transmitting, by the transceiver, an electromagnetic transmit
signal; guiding, by the probe, the transmit signal towards and into
the product; returning, by the probe, an electromagnetic reflection
signal resulting from reflection of the transmit signal at the
surface of the product and at the second probe end; receiving, by
the transceiver, the reflection signal; determining, based on the
reflection signal and a timing relation between the reflection
signal and the transmit signal, an echo signal exhibiting an echo
signal strength as a function of a propagation parameter indicative
of position along the probe; and determining, by the processing
circuitry, the level of the surface of the product based on a
propagation parameter value indicative of a first threshold
position along the probe for which the echo signal has reached a
predetermined threshold signal strength, and an offset indicative
of an offset distance along the probe from the first threshold
position towards the second probe end.
[0009] The "transceiver" may be one functional unit capable of
transmitting and receiving electromagnetic signals, or may be a
system comprising separate transmitter and receiver units.
[0010] The tank may be any container or vessel capable of
containing a product, and may be metallic, or partly or completely
non-metallic, open, semi-open, or closed.
[0011] The propagation parameter may be any parameter indicative of
a position along the probe. For example, the propagation parameter
may be any one of a time-of-flight of the reflection signal, a
distance from a reference position at the first probe end, and a
level in the tank, etc.
[0012] The present invention is based upon the realization that the
lower dead zone can be reduced or even avoided without a highly
inductive probe termination if the level of the surface of the
product can be determined from a composite peak in the echo signal
formed by a combination of the echo signal from reflection at the
surface of the product and the echo signal from reflection at the
second end of the probe.
[0013] The present inventors have further realized that this can be
achieved by determining the level of the surface of the product, at
least when the level is close to the second probe end, based on the
position along the probe where the echo signal strength reaches a
predetermined threshold value, and an offset distance from that
position towards the second probe end.
[0014] Hereby, the probe termination arrangement can be made more
robust, since the inductance can be lower without sacrificing the
ability to reduce or avoid the lower dead zone. This may make the
radar level gauge system less sensitive to damage and disturbances,
and therefore suitable for a greater range of applications.
Furthermore, the requirements on the manufacturing tolerances of
the probe can be reduced, resulting in simpler and more
cost-efficient manufacturing and/or installation at the tank.
[0015] The offset may advantageously be a predetermined value or
may be selected among a set of predetermined values based on at
least one measured property, such as a temperature, or a system
specific property. Furthermore, the offset may depend on at least
one material property of the second substance, such as the
dielectric constant of the second substance. For example, the
offset may be determined based on an estimated echo signal
indicative of reflection of the transmit signal at the surface of
the product only. Alternatively, or in combination, the offset may
be determined based on one or several echo signals resulting from
reflection of the transmit signal at the surface of the product
when the surface of the product is sufficiently separated from the
second probe end for the reflection at the surface of the product
to result in an isolated peak in the echo signal. Such a measured
isolated peak in the echo signal can be used to establish a
mathematical model of the peak. The mathematical model, which may
be simple (as will be described further below) or more complex can
be used to determine the offset for a given threshold signal
strength.
[0016] According to embodiments, furthermore, the transmit signal
may comprise a first pulse train having a first pulse repetition
frequency; and the method may further comprise the steps of:
generating, by the transceiver, an electromagnetic reference signal
in the form of a second pulse train having a second pulse
repetition frequency controlled to differ from the first pulse
repetition frequency by a frequency difference; and the echo signal
may be determined based on the reflection signal, the reference
signal, and the frequency difference.
[0017] The pulses in the first pulse train may advantageously be
so-called DC-pulses.
[0018] It should be noted that the steps of methods according to
embodiments of the present invention need not necessarily be
carried out in any particular order, unless explicitly or
implicitly required.
[0019] According to a second aspect of the present invention, it is
provided a radar level gauge system for determining a level of a
product in a tank, the radar level gauge system comprising: a
transceiver for generating, transmitting and receiving
electromagnetic signals; a probe arranged to extend towards and
into the product from a first probe end coupled to the transceiver
to a second probe end, the probe comprising a first probe conductor
and a second probe conductor being conductively coupled to each
other by a probe termination arrangement at the second probe end;
echo signal forming circuitry connected to the transceiver for
forming, based on the reflection signal and a timing relation
between the reflection signal and the transmit signal, an echo
signal exhibiting an echo signal strength as a function of a
propagation parameter indicative of position along the probe; and
level determining circuitry connected to the echo signal forming
circuitry for determining the level of the product in the tank
based on a propagation parameter value indicative of a first
threshold position along the probe for which the echo signal has
reached a predetermined threshold signal strength, and an offset
indicative of an offset distance along the probe from the first
threshold position towards the second probe end.
[0020] According to embodiments, the probe termination arrangement
may provide an inductance between the first probe conductor and the
second probe conductor being higher than about 1 nH and lower than
about 30 nH.
[0021] By configuring the probe termination arrangement to provide
an inductance in the above range, a favorable trade-off between
robustness and reduction in the dead zone at the second probe end
can be achieved.
[0022] Further embodiments of, and effects obtained through this
second aspect of the present invention are largely analogous to
those described above for the first aspect of the invention.
[0023] In summary, the present invention thus relates to a method
of determining a level of a product in a tank, comprising
generating and transmitting an electromagnetic transmit signal;
guiding the transmit signal towards and into the product; returning
an electromagnetic reflection signal resulting from reflection of
the transmit signal; receiving, the reflection signal; determining,
based on the reflection signal and a timing relation between the
reflection signal and the transmit signal, an echo signal
exhibiting an echo signal strength as a function of a propagation
parameter indicative of position along the probe; and determining
the level of the surface of the product based on a propagation
parameter value indicative of a first threshold position along the
probe for which the echo signal has reached a predetermined
threshold signal strength, and an offset indicative of an offset
distance along the probe from the first threshold position towards
the second probe end.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] These and other aspects of the present invention will now be
described in more detail, with reference to the appended drawings
showing example embodiments of the invention, wherein:
[0025] FIG. 1 schematically illustrates an exemplary tank
arrangement comprising a radar level gauge system according to an
embodiment of the present invention;
[0026] FIG. 2 is schematic illustration of the measurement unit
comprised in the radar level gauge system in FIG. 1;
[0027] FIG. 3 is a partial schematic block diagram of the radar
level gauge system according to an embodiment of the present
invention;
[0028] FIG. 4 is a flow-chart schematically illustrating an example
embodiment of the method according to the present invention;
[0029] FIG. 5A schematically illustrates examples of the transmit
signal, the reflection signal and the reference signal;
[0030] FIG. 5B is a partial enlarged view of a portion of the
transmit signal and the reference signal in FIG. 5A;
[0031] FIGS. 6A-B schematically illustrate the echo signal
resulting from time-correlation of the surface reflection signal
and the reference signal in FIG. 4A for an example situation where
the surface of the product in the tank is close to the second end
of the probe; and
[0032] FIGS. 7A-D show example configurations of the probe
termination arrangement comprised in the radar level gauge system
in FIG. 1.
DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT OF THE INVENTION
[0033] FIG. 1 schematically shows a level measuring system 1
comprising a radar level gauge system 3 according to an example
embodiment of the present invention, and a host system 5
illustrated as a control room.
[0034] The radar level gauge system 3, which is of GWR (Guided Wave
Radar) type, is arranged at a tank 7 having a tubular mounting
structure 9 (often referred to as a "nozzle") extending
substantially vertically from the roof of the tank 7.
[0035] In the present exemplary measurement situation, the tank 7
contains a product 11 and a tank atmosphere 13 above the product
11. The tank atmosphere 13 may be air or vapor, and the product 11
may, for example, be oil or another liquid through which
electromagnetic signals can be guided by a probe.
[0036] The radar level gauge system 3 is installed to measure the
level of the surface 15 of the product 11. The radar level gauge
system 3 comprises a measuring electronics unit 17 arranged outside
the tank 7, and a probe 19, extending from a first probe end 21
coupled to the measuring electronics unit 17, through the tubular
mounting structure 9, towards and into the product 11, to a second
probe end 23 at the bottom of the tank 7. In the example
measurement situation in FIG. 1, the surface 15 of the product 11
is indicated as being close to the second probe end 23, at a level
that may be inside the so-called lower dead zone or blind zone for
various existing radar level gauge systems.
[0037] As is schematically indicated in FIG. 1, in particular in
the enlarged schematic functional view from the second end 23 of
the probe 19, the probe 19 has a first probe conductor 25, a second
probe conductor 27, and a probe termination arrangement 29
conductively coupling the first probe conductor 25 to the second
probe conductor 27.
[0038] In the example embodiment in FIG. 1, the probe 19 is shown
in the form of a large coaxial probe with the first probe conductor
25 being an inner conductor and the second probe conductor 27 being
a coaxially arranged outer conductor. It should, however, be noted
that the probe 19 may alternatively be any other kind of probe
comprising first 25 and second 27 probe conductors, such as a twin
line transmission line probe, with parallelly extending wires or
rods, or an "ordinary" coaxial probe with a smaller diameter of the
outer conductor (and the inner conductor) than the large coaxial
probe in FIG. 1. Furthermore, while the probe termination
arrangement 29 is conceptually indicated in FIG. 1, the skilled
person will realize that there are many possible ways of
implementing the probe termination arrangement 29. Some
representative examples of probe termination arrangements that may
be suitable for various embodiments of the radar level gauge system
3 will be described further below with reference to FIGS. 7A-D.
[0039] In operation, an electromagnetic transmit signal S.sub.T is
transmitted and guided by the probe 19 towards and into the product
11. An electromagnetic reflection signal S.sub.R is returned, by
the probe 19. Based on the reflection signal and a timing relation
between the reflection signal and the transmit signal, the
measurement unit 17 can determine the level of the surface 15. The
radar level gauge system in FIG. 1 will now be described in more
detail with reference to the schematic block diagram in FIG. 2.
[0040] Referring to the schematic block diagram in FIG. 2, the
measurement unit 6 of the radar level gauge system 2 in FIG. 1
comprises a transceiver 31, a measurement control unit (MCU) 33, a
wireless communication control unit (WCU) 35, a communication
antenna 37, an energy store, such as a battery 39, and a
feed-through 41 between the outside and the inside of the tank
7.
[0041] As is schematically illustrated in FIG. 2, the MCU 33
controls the transceiver 31 to generate, transmit and receive
electromagnetic signals. The transmitted signals pass through the
feed-through 31 to the inner probe conductor 25 of the probe 19,
and the received signals pass from the probe 19 through the
feed-through 41 to the transceiver 31.
[0042] The MCU 33 may determine the level of the surface 15 of the
product 11, and provide a value indicative of the level to an
external device, such as the control center 5 in FIG. 1, from the
MCU 33 via the WCU 35 through the communication antenna 37. The
radar level gauge system 1 may, for example, be configured
according to the so-called WirelessHART communication protocol (IEC
62591).
[0043] Although the measurement unit 17 is shown to comprise an
energy store 39 and to comprise devices (such as the WCU 35 and the
communication antenna 37) for allowing wireless communication, it
should be understood that power supply and communication may be
provided in a different way, such as through communication lines
(for example 4-20 mA lines).
[0044] The local energy store need not (only) comprise a battery,
but may alternatively, or in combination, comprise a capacitor or
super-capacitor.
[0045] The radar level gauge system 3 in FIG. 1 will now be
described in greater detail with reference to the schematic block
diagram in FIG. 3.
[0046] Referring now to FIG. 3, there is shown a more detailed
block diagram of the exemplary transceiver 31 in FIG. 2.
[0047] As is schematically shown in FIG. 3, the transceiver 31
comprises a transmitter branch for generating and transmitting the
transmit signal S.sub.T, and a receiver branch for receiving and
operating on the reflection signal S.sub.R. As is indicated in FIG.
3, the transmitter branch and the receiver branch are both
connected to a directional coupler 41 to direct signals from the
transmitter branch to the probe 19 and to direct reflected signals
being returned by the probe 19 to the receiver branch.
[0048] As is schematically indicated in FIG. 3, the transceiver 31
comprises pulse generating circuitry, here in the form of a first
pulse forming circuit 43, a second pulse forming circuit 45, and a
timing control unit 47 for controlling the timing relationship
between the transmit signal output by the first pulse forming
circuit 43 and the frequency shifted reference signal S.sub.REF
output by the second pulse forming circuit 45.
[0049] The transmitter branch comprises the first pulse forming
circuit 43, and the receiver branch comprises the second pulse
forming circuit 45 and measurement circuitry 49.
[0050] As is schematically indicated in FIG. 3, the measurement
circuitry 49 comprises a time-correlator, here in the form of a
mixer 51, a sample-and-hold circuit 53 and amplifier circuitry 55.
In embodiments of the present invention, the measurement circuitry
49 may further comprise an integrator 57.
[0051] Additionally, as was briefly described above with reference
to FIG. 2, the radar level gauge system 3 comprises processing
circuitry 33 that is connected to the measurement circuitry 49 for
determining the level of the surface 15 of the product 11 in the
tank 7.
[0052] When the radar level gauge system 3 is in operation to
perform a filling level determination, a time correlation is
performed in the mixer 51 between the reflection signal S.sub.R and
the reference signal S.sub.REF that is output by the second pulse
forming circuit 45. The reference signal S.sub.REF is a pulse train
with a pulse repetition frequency that controlled to differ from
the pulse repetition frequency of the transmit signal S.sub.T, by a
predetermined frequency difference .DELTA.f. When a measurement
sweep starts, the reference signal S.sub.REF and the transmit
signal S.sub.T are in phase, and then parameter values indicative
of a time correlation between the reference signal and the
reflected signal S.sub.R are determined to form an echo signal,
together with the frequency difference .DELTA.f. Based on an
analysis of the echo signal, level of the surface 15 of the product
11 in the tank 7 can be determined, as will be described further
below.
[0053] The time-expansion technique that was briefly described in
the previous paragraph is well known to the person skilled in the
art, and is widely used in pulsed radar level gauge systems.
[0054] As is clear from the above discussion, the output from the
mixer 51 will be a sequence of values, where each value represents
a time correlation between a pulse of the reference signal
S.sub.REF and the reflection signal S.sub.R. The values in this
sequence of values are tied together to form a continuous signal
using the sample-and-hold circuit 53.
[0055] In this context it should be noted that the sample-and-hold
circuit 53 is simply an illustrative example of a device capable of
maintaining a voltage level over a given time, and that there are
various other devices that can provide the desired functionality,
as is well known to the person skilled in the art.
[0056] In the example embodiment of FIG. 3, the time-correlated
signal--the correlation signal S.sub.C--output from the
sample-and-hold circuit 53 is provided to an integrator to form a
measurement signal S.sub.M, which is amplified by the low noise
amplifier LNA 55. The above-mentioned echo signal can be formed, by
echo signal forming circuitry 59, based on the measurement signal
S.sub.M and the frequency difference .DELTA.f. The filling level of
the product 11 (the level of the surface 15) can, according to
embodiments of the present invention, be determined by the level
determining circuitry 61. Along a segment of the probe 19 that is
neither close to the first 21 nor the second 23 probe end, the
filling level may be determined using conventional methods.
[0057] According to example embodiments of the present invention,
the filling level close to the second probe end 23 may be
determined in accordance with the method described below, with
reference to the schematic flow-chart in FIG. 4 and further
reference to other figures as indicated.
[0058] In step 401, the transmit signal S.sub.T is generated as a
pulse train of transmit pulses, and transmitted by the transceiver
31.
[0059] In step 402, taking place at the same time as step 401, the
reference signal S.sub.REF is generated as a pulse train of
reference pulses.
[0060] In step 403, the transmit signal S.sub.T is guided by the
probe 19 towards and into the product 11 in the tank 7.
[0061] In step 404, the reflection signal S.sub.R resulting from
reflection of the transmit signal S.sub.T at impedance transitions
encountered thereby is returned by the probe 19 and received by the
transceiver 31. Notably, the impedance transitions encountered by
the transmit signal S.sub.T include impedance transitions provided
by the surface 15 of the product 11 and the probe termination
arrangement 29 at the second probe end 23. For further illustration
of the above-described steps 401 to 404, FIGS. 5A-B are now
referred to.
[0062] FIG. 5A is a simplified timing diagram schematically showing
the relative timing of the transmit signal S.sub.T, the reflected
signal S.sub.R, and the reference signal S.sub.REF according to an
example embodiment of the invention.
[0063] As is schematically indicated in FIG. 5A, the transmit
signal S.sub.T, formed by transmit pulses 63, and the reference
signal S.sub.REF, formed by reference pulses 65, are controlled by
the timing control unit 47 to be in phase at the start of the
measurement sweep. A full measurement sweep may typically be
defined by the difference frequency .DELTA.f, since the transmit
signal S.sub.T and the reference signal S.sub.REF, in this
particular example, need to be in phase at the start of a new
measurement sweep. As is also schematically indicated in FIG. 5A,
the reflection signal S.sub.R here comprises a first set of
reflection pulses 67 resulting from reflection of the transmit
pulses 63 at the surface 15 of the product 11, and a second set of
reflection pulses 69 resulting from reflection of the transmit
pulses 63 by the impedance transition provided by the probe
termination arrangement 29 at the second probe end 23. Each of the
first 67 and second 69 set of reflection pulses has the same pulse
repetition frequency as the transmit signal S.sub.T, but lags
behind the transmit signal S.sub.T with a time corresponding to the
time-of-flight indicative of the electrical distance to the surface
15 of the product and the probe termination arrangement 29,
respectively.
[0064] The reference signal S.sub.REF is initially in phase with
the transmit signal S.sub.T, but will, due to its lower pulse
repetition frequency "run away from" the transmit signal S.sub.T
and "catch up with" the surface reflection signal S.sub.R.
[0065] When the time-varying phase difference between the transmit
signal S.sub.T and the reference signal S.sub.REF corresponds to
the time-of-flights of the reflection pulses of the reflection
signal S.sub.R, there will be a time-correlation between pulses of
the reference signal S.sub.REF and pulses of the surface reflection
signal S.sub.R. This time-correlation results in a time-expanded
correlation signal S.sub.C, which can, in turn, be converted to a
measurement signal S.sub.M.
[0066] Example waveforms of the transmit pulses 63 and the
reference pulses 65 are provided in the schematic magnified view in
FIG. 5B.
[0067] Returning to the flow-chart in FIG. 4, the echo signal is
determined, in step 405, by the echo signal forming circuitry 59,
based on the reflection signal and a timing relation between the
reflection signal and the transmit signal. For example, the echo
signal may advantageously be determined based on the
above-mentioned time-expanded measurement signal S.sub.M and the
frequency difference .DELTA.f.
[0068] The above thorough explanation was provided for the case of
a so-called pulsed measurement technique. The echo signal may
alternatively be determined using other techniques, in which a
frequency modulated transmit signal is used, as will be apparent to
those of skilled in the art of radar level gauging.
[0069] An example of the echo signal, for an exemplary measurement
situation where the surface 15 of the product 11 is close to the
second probe end 23 of the probe 19, will now be described with
reference to FIGS. 6A-B.
[0070] FIG. 6A schematically shows an echo signal 71 exhibiting an
echo signal strength (or amplitude) as a function of a propagation
parameter indicative of position along the probe 19. In this case,
the chosen propagation parameter is position z along the probe in
relation to a reference position at the first probe end 21. FIG. 6B
is an enlarged view of a portion of the echo signal 71 indicating
reflection by the impedance transitions provided by the surface 15
of the product 11 and the probe termination arrangement 29 at the
second probe end 23 of the probe 19.
[0071] As is schematically shown in FIG. 6A, the echo signal 71
indicates a reference echo 73 resulting from reflection of the
transmit signal S.sub.T at a reference impedance transition (such
as the feed-through 41) at the first probe end 21, and a composite
peak 75 formed by a combination of the echo signals from reflection
at the impedance transitions provided by the surface 15 of the
product 11 and the probe termination arrangement 29 at the second
probe end 23.
[0072] As is schematically shown in FIGS. 6A-B, the composite peak
75 is a broad and asymmetrical echo peak that only exhibits a
single local extremum (maximum) 77, so that the surface 15 of the
product 11 and the second probe end 23 cannot be distinguished
based on conventional peak detection. If conventional peak
detection were used, the product level 15 in this example would be
considered to be in the lower dead zone.
[0073] Returning to the flow-chart in FIG. 4, the level of the
surface 15 of the product 11 in the tank 7 is instead determined
using the procedure described below.
[0074] In step 406, a first threshold position z.sub.TH1 along the
probe for which the echo signal 71 has reached a predetermined
threshold signal strength TH is determined.
[0075] The first interface level is then determined, in step 407,
based on the first threshold position z.sub.TH1 and a predetermined
offset distance .DELTA.z along the probe 19 from the first
threshold position z.sub.TH1 towards the second probe end 23.
[0076] The predetermined offset distance .DELTA.z is determined
based on a model of the expected reflection of the transmit signal
S.sub.T at the surface 15 of the product 11 only, and/or on
previous test measurements. The echo pulse shape of the reflection
at the surface 15 can be calculated based on known propagation
properties of the probe 19 and the dielectric constants of the tank
atmosphere 13 and the product 11 in the tank 7.
[0077] For the case where the tank atmosphere 13 is air, the
product 11 is oil, and the probe 19 is an exemplary coaxial probe,
the shape of the echo pulse 79 from reflection at the surface 15
only can be approximated by the general curve shape expression:
f(x)=SummitAmplitude(1-Qx.sup.2),
[0078] where Q.apprxeq.100.
[0079] It should be noted that the value of Q depends on the
particular configuration of the radar level gauge system 3, and
that it may be temperature dependent. For an example configuration,
the Q-value may be selected from values in the range 80-120,
depending on the temperature.
[0080] This means that the offset distance .DELTA.z can be
determined according to the following relation:
.DELTA.z = 1 - TH SummitAmplitude Q ##EQU00001##
[0081] The position along the probe 19 of the surface 15 of the
product 11 in relation to the reference impedance transition (such
as the feed-through 41) then becomes:
z.sub.1=z.sub.TH+.DELTA.z
[0082] The level of the surface 15 can be determined based on the
position z.sub.1 (distance along the probe 19 from the reference
impedance transition), and the known position of the reference
position impedance (such as the feed-through 41).
[0083] FIGS. 7A-D show example configurations of the probe
termination arrangement 29 comprised in the radar level gauge
system in FIG. 1. A suitable probe termination arrangement 29
should be easy to mount to the probe 19 at the second probe end 23,
and it should be mechanically and electrically robust. In
particular, it should maintain its electrical properties even if
subjected to harsh environments and vibrations etc. Advantageous
electrical properties for a substantial reduction of the lower dead
zone may be that the probe termination arrangement 29 provides an
inductance between the first probe conductor 25 and the second
probe conductor 27 being higher than 1 nH. To keep the probe
termination arrangement 29 as mechanically robust as desired, it
may be beneficial to configure the probe termination arrangement 29
to provide an inductance below about 30 nH. The different exemplary
probe termination arrangement configurations shown in FIGS. 7A-D
all provide an inductance of about 5-15 nH when installed in a
"Large Coaxial Probe" with an outer diameter of the outer conductor
27 being 42 mm.
[0084] The first example configuration of the probe termination
arrangement 29 shown in FIG. 7A comprises an electrically
conductive member 81 that is attached to the first probe conductor
25 and to the second probe conductor 27. In this first example
configuration, the electrically conductive member is provided in
the form of a metal sleeve that is conductively and mechanically
connected to the first probe conductor 25 and the second probe
conductor 27 by inserting a nut 83 in the first probe conductor 25,
passing a bolt 85 through a hole in the second probe conductor 27,
the metal sleeve, and a hole in the first probe conductor 25, and
joining the bolt 85 and the nut 83 to press the metal sleeve
between the outer surface of the inner conductor 25 and the inner
surface of the outer conductor 27.
[0085] The second example configuration of the probe termination
arrangement 29 shown in FIG. 7B comprises an electrically
conductive member that is attached to the first probe conductor 25
and to the second probe conductor 27. In this second example
configuration, the electrically conductive member 81 is provided in
the form of a metal sleeve accommodating the first probe conductor
25. The metal sleeve is conductively and mechanically connected to
the first probe conductor 25 by a fixing screw 89 (inside the hole
in FIG. 7B) and to the second probe conductor 27 by a screw 91. To
allow for bigger tolerances in manufacturing and/or assembly, the
longitudinal extension of the electrically conductive member 81
(metal sleeve) may be at least 10 mm.
[0086] In the third example configuration of the probe termination
arrangement 29 shown in FIG. 7C, the electrically conductive member
81 is conductively and mechanically connected to the first probe
conductor 25 by a bolt 93 and to the second probe conductor 27 by a
rivet 95.
[0087] In the third example configuration of the probe termination
arrangement 29 shown in FIG. 7D, the electrically conductive member
81 is conductively and mechanically connected to the first probe
conductor 25 by a first weld 97 and to the second probe conductor
27 by a second weld 99.
[0088] The person skilled in the art realizes that the present
invention by no means is limited to the preferred embodiments
described above. For example, other probe configurations and other
substances in the stratified substance composition may result in
different selections of the threshold signal strengths and
different estimations of the offset distances.
* * * * *